class SingleDog {
    show() {
      console.log('我是一个单身狗');
    }
    static getInstance() {
      if (!SingleDog.instance) {
        SingleDog.instance = new SingleDog();
      }
      return SingleDog.instance;
    }
  }
  
  const s1 = SingleDog.getInstance()
  const s2 = SingleDog.getInstance()
  
  console.log(s1 === s2);
  